WO 2006/095320 discloses a method for detecting the location, shape and size of at least one object placed on a plane within the touch sensor boundaries of a touch screen. The touch screen display includes on its periphery, N light transmitters and M sensors. The method is generally comprised of two stages: a calibration stage and an operational stage. During the calibration stage, as each light transmitter is switched on during its respective turn-on time, the light beam that is cast may be detected by certain ones of the sensors. For each light transmitter, the identification of the sensors that detect the respective light transmitter's light beam is recorded as calibration data. Non-calibration data is recorded over a cycle of operation in the presence of an object. A comparison is made of the calibration data and non-calibration data allowing the shadow area cast by the object to be determined. In certain embodiments, the overall measurement accuracy may be increased by increasing the density of transmitters and/or receivers in certain areas of the screen where detection proves to be less accurate than other areas. This non-even configuration of transmitters and/or receivers can compensate for the less accurate detection.
The non-even configuration is less appropriate for remedying a decrease in accuracy caused by one object casting a shadow on another object to be detected.